1. Field of the Invention
The present invention relates to a heat exchanger for an air conditioner, and more particularly to a heat exchanger which provides an improved heat transfer performance resulting from the turbulence and mixture of the air currents that flow in spaces between a plurality of flat fins.
2. Description of the Prior Art
A conventional heat exchanger for an air conditioner includes, as shown in FIG. 1, a plurality of flat fins 1 arranged in a parallel relation to each other at predetermined intervals and a plurality of heat exchanging tubes 2 passing through the fins 1 perpendicular thereto. The air currents flow in the space defined between the fins 1 in the direction of the arrow in FIG. 1 and exchange heat with the fluid flowing in the heat exchanging tubes 2.
For a thermal fluid flowing across each flat fin 1, it has been known that the thickness of the thermal boundary layer 3 on both heat transfer surfaces of the fin 1 is gradually increased in proportion to the square root of the distance from the air current inlet end of the fin 1 as shown in FIG. 2. In this regard, the heat transfer rate of the fin 1 is remarkably reduced in proportion to the distance from the air current inlet end. Therefore, the above heat exchanger has a lower heat transfer efficiency.
For the thermal fluid flowing across each heat transfer pipe 102, it has been also known that, when lower velocity air currents flow in the direction of the arrow of FIG. 3, the air currents separate from the outer surface of the pipe 2 at portions spaced apart from the center point of outer surface of the pipe 4 at angles of 70-degree to 80-degree. Therefore, a dead air region 4 is formed behind each tube 2 in a direction of the air flow as shown by the hatched region of FIG. 3. In the dead air region 4, the heat transfer rate of the tube 2 is remarkably reduced so that the heat transfer efficiency of the above heat exchanger becomes worse.
In order to overcome the above problems, there has been proposed another solution as disclosed in U.S. application Ser. No. 08/890,562 filed on Jul. 9, 1997. This heat exchanger, as shown in FIGS. 4 and 5, includes a plurality of heat exchanging tubes 2 which are fitted into the regularly spaced flat fins 1 such that the tubes 2 are perpendicular to the fins 1.
The heat exchanger also includes a plurality of angled louver patterns which are formed adjacent the tubes 2 passing through each fin 1. The patterns located between the tubes 2 comprise: a first angled louver pattern 20 and a second louver pattern 30 inclined opposite to the first louver pattern 20. Those patterns are located in the left (upstream) half area of respective tubes 2, and include louvers projecting from both surfaces of the flat fin 1, such that air current flowing through the patterns 20, 30 becomes turbulent and mixed. A third angled louver pattern 40 and a fourth louver pattern 50 inclined opposite to the third louver pattern, are located in the right (downstream) half area of respective tubes 2, and include louvers projecting from both surfaces of the flat fin 1, such that air current flowing through those patterns 40, 50 becomes again turbulent and mixed, resulting in a reduction of the dead air region. Those louver patterns 20-50 are radially positioned around each tube 2.
Also, the angled first and second louver patterns 20 and 30 are placed in mirror image relationship to each other such that the air currents flowing over both surfaces of the flat fin 1 and in an upstream half area between two tubes 2 result in a turbulent flow and mixing. Further, the angled third and fourth louver patterns 40 and 50 are similarly placed in mirror image relationship to each other such that the resultant air current having passed the patterns 20 and 30 continues to pass the remaining half area between the tubes 2 and becomes turbulently mixed, thereby reducing the dead air region.
Each of the first and second louver patterns 20, 30 includes strips or louvers 70, each of which has a left (upstream) end 76 (see FIG. 5) projecting from a first surface 1A of the flat fin 1 and a right (downstream) end 78 projecting from a second surface 1B of the flat fin 1, respectively. Each louver provides a slit extending transversely relative to the air flow. The louvers according to the present invention may be formed by way of a cutting and twisting process. The third and fourth louver patterns 40, 50 are similar to those first and second louver patterns 20, 30, but the upstream louvers thereof project from the second surface of the fin rather than from the first surface.
A generally round base portion 60 of the fin occupies an area defined between upper ends of the first and third louver patterns 20, 40 and a lower outer circumference of a respective tube 2. With the base portion 60 interposed therebetween and concentrically relative to the tube 2, the first and third louver patterns 20, 40 are radially placed around the tube 2. Similarly, the second and fourth louver patterns 30, 50 are radially placed to face an upper outer circumference of the next lower tube 2, with a rounded base portion 60A interposed therebetween.
The first and third louver patterns 20, 40 and the second and fourth louver patterns 30, 50 are symmetrical to each other, and are separated by a base portion 60B of the fin.
The louvers 70 to 75 included in the respective patterns 20, 30, 40, 50 are sequentially arranged without there being any fin base portion therebetween and are directly formed by way of a cutting and twisting process.
In the drawings, reference numeral 80 denotes solid beads each of which extends perpendicularly to the air flow and lies in a plane PL containing the center axis of two adjacent tubes 2. The beads formed by way of the beading process serve to: drain condensed water (i.e., dew) that may be generated from the heat exchanging tubes 2, reinforce the flat fin 1, and enlarge the surface area of the flat fin 1.
The bead 80 is located in a base portion 60C disposed between the first and second patterns 20, 30 and the third and fourth patterns 40, 50.
A single bead is configured to project above the second surface 1A of the flat fin 1, and thus has a pattern symmetrical relative to a central longitudinal axis of the bead, i.e., an axis lying in the plane PL. Upstream and downstream halves 80A, 80B of the bead 80 are symmetrically bent at a suitable angle, to form an inverted V-shape as can be seen in FIG. 5.
However, for the conventional heat exchanger as described above, the first to fourth louver patterns 20 to 50, each placed at predetermined positions of the flat fin 1, have many louvers 70 to 75, e.g., as many as six in number, respectively, causing the height H and width W of each louver to be lower and narrower. Further, this results in a wide angle of 49-degrees defined between two louver patterns 30 and 50 (see FIG. 4), these patterns being horizontally symmetrical to each other, as shown in FIGS. 4 and 5.
Accordingly, at upstream and downstream ends of each tube 2, there is an area with a relatively large vertical width L in which no louvers can be formed. This acts as a cause of greatly reduced heat transfer efficiency, in spite of a lower degree of the pressure drop, as well as producing a slower rate of air current at those areas. There occurs problems of an insufficient turbulence of air current and a heat transfer efficiency remarkably reduced.